Pivotal bone anchor assembly with preloaded articulating retainer and bottom loaded shank

ABSTRACT

A pivotal bone anchor assembly includes a receiver having central bore with a partially spherical engagement surface adjacent a lower opening, and a shank having a capture portion at a proximal end that is uploadable into the central bore of the receiver through the lower opening. The assembly also includes a retainer comprising a compressible, open ring-shaped body having at least a slit or missing section, a center opening, and an outer partially spherical surface configured for pivotal engagement with the partially spherical engagement surface of the receiver. The assembly further includes an insert comprising a center aperture configured to provide access to the internal drive socket for the drive tool, an upper surface configured to engage the fixation rod, and a lower surface configured to transfer the downwardly-directed force toward the capture portion of the shank to lock the assembly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/655,239 filed Feb. 22, 2005.

BACKGROUND OF THE INVENTION

The present invention is directed to polyaxial bone screws for use inbone surgery, particularly spinal surgery.

Bone screws are utilized in many types of spinal surgery, such as forosteosynthesis, in order to secure various implants to vertebrae alongthe spinal column for the purpose of stabilizing and/or adjusting spinalalignment. Although both closed-ended and open-ended bone screws areknown, open-ended screws are particularly well suited for connections torods and connector arms, because such rods or arms do not need to bepassed through a closed bore, but rather can be Laid or urged onto theopen receiver channel of an open ended bone screw.

Typical open-ended bone screws include a threaded shank with a pair ofparallel projecting branches or arms which form a yoke with a U-shapedslot or channel to receive a rod. Hooks and other types of connectors,as are used in spinal fixation techniques, may also include open endsfor receiving rods or portions of other structure.

A common mechanism for providing vertebral support is to implant bonescrews into certain bones which then in turn support a longitudinalstructure such as a rod, or are supported by such a rod. Bone screws ofthis type may have a head or receiver that receives the rod or otherstructure that is fixed relative to a shank thereof. In the fixed bonescrews, the fixed receiver cannot be moved relative to the shank and therod must be favorably positioned in order for it to be placed within thereceiver. This is sometimes very difficult or impossible to do.Therefore, polyaxial bone screws are commonly preferred.

Open-ended polyaxial bone screws allow rotation of a rod receiver withrespect to the shank until a desired rotational position of the receiveris achieved relative to the shank. A rod is inserted into the receiverand eventually the receiver is locked or fixed in a particular positionrelative to the shank.

There are a variety of ways in which the rod may be captured within anopen polyaxial bone screw. Some sort of closure structure or plug isrequired so as to block the channel opening once the rod is insertedtherein and, also preferably urge the rod into a seated and lockedposition relative to the receiver. A substantial amount of torque isrequired to seat the plug against the rod which in turn seats the rod inthe receiver channel so as to prevent relative motion between the rodand the bone screw. Consequently, the need to highly torque a plugdisposed between the arms of an open bone screw functions counter to theneed to prevent the bone screw arms from splaying.

Certain prior art plug type closures have been threadably receivedbetween the opposed arms of the bone screw receiver using conventionalV-shaped thread forms which has resulted in a significant amount ofradially outward pressure or force being applied to the arms of the bonescrew receiver. Such outward force may result in splaying of the arms,after which the closure becomes loose which may either result in afailure of the implant by allowing the rod to slip relative to the bonescrew or the closure may even come completely out of the receiver of thebone screw for total failure of the implant. In order to help relievethis problem, certain of the prior art has added structure to the rodengaging lower surface of the closure. Such structure has includedadding a central or axial point or ring designed to penetrate into therod and help lock the rod into place. Surface finish on the plug, suchas knurling, has also been utilized.

At a side of the rod opposite of the closure plug, various compressionspacers or insertable compression structures have been developed thatare operably disposed adjacent the rod and within the bone screwreceiver. Such compression structures have been used to frictionallylink the rod with the bone screw shank and to aid in snugly seating therod in the open bone screw, thus aiding in preventing relative motionbetween the rod and the bone screw. Prior art bone screw compressioninserts have typically been utilized with top-loaded bone screw shanks,having substantially spherical heads that are integral with the shankbody. Such compression inserts include those that contact an upperspherical portion of the bone screw shank and others that extendsubstantially around such a spherical surface. Such compression insertsmay also include a curved upper surface or surfaces for receiving therod.

Bone screw compression inserts may desirably reduce relative motionbetween the rod and the bone screw, but may be undesirable in practiceas they may also require separate insertion during surgery, afterimplantation of the bone screw shank, and may be small and thusdifficult to handle. Alternatively, compression inserts loaded in a bonescrew prior to implantation may obstruct bone screw features utilizedfor driving the threaded bone screw shank into bone, or require lessthan desirable modifications in the bone screw, decreasing strengthand/or requiring specialized driving tools.

SUMMARY OF THE INVENTION

A polyaxial bone screw assembly according to the invention includes ashank having an upper portion and a body for fixation to a bone. Theshank upper portion has a width or diameter smaller than a diameter of alower opening of a cooperating receiver that also includes an openchannel for receiving a rod or other elongate structure. The assemblyfurther includes an independent non-integral retaining and articulatingstructure for attachment to the shank upper portion within the receiver.Furthermore, a compression structure is operably disposed between theretaining and articulating structure and the rod. The shank is connectedto the receiver by the retaining and articulating structure that isoperably slidably mated with an inner surface of the receiver, allowingthe shank body to be swivelable with respect to the receiver.

According to one aspect of the invention, a closure member having aflangeform thereon is mateable with cooperating flange-form structure oninner arms of an upper portion of the receiver. The closure memberfurther includes a dome-shaped lower surface for operably pressingagainst the rod or other structural member. The rod in turn contacts andpresses on the compression structure and the compression structurecontacts and presses on the retaining and articulating structure whichfixes the retaining and articulating structure against an inner seatingsurface of the receiver.

According to another aspect of the invention, a bone screw shank upperportion is sized and shaped to be insertable through a lower opening ofa receiver. The shank upper portion includes a helical thread and insome embodiments a lateral projection. In a particular embodiment theprojection is in the form of a cylinder forming a buttress stop. Theretaining and articulating structure includes a through-bore defined inpart by a helical thread sized and shaped to mate with the helicalthread of the shank capture structure. The retaining and articulatingstructure also includes structure, such as a cooperating buttress stopsized and shaped to abut against the projection when fully installed onthe shank upper portion, stopping the retaining and articulatingstructure from further rotation down the shank upper portion. Theretaining and articulating structure could have a vertical slit and thusbe down-loadable or up-loadable into the receiver.

According to another aspect of the invention, the shank upper portionthat is insertable in the receiver lower opening includes a drivingformation at a top surface thereof. The driving formation is sized andshaped to receive an end of a driving tool. A further aspect of theinvention includes a compression structure or insert that ispre-loadable in the bone screw receiver and includes a central throughbore, allowing for the driving of the shank into bone with the insertloaded in the bone screw receiver. The insert in some embodiments can bedown-loaded or up-loaded into the receiver, in particular the insert andthe retaining structure could both be up-loaded into the receiver.

Objects and Advantages of the Invention

Therefore, objects of the present invention include: providing animproved spinal implant assembly for implantation into vertebrae of apatient; providing such an assembly that includes a receiver with anopen channel, a shank pivotally connected to the receiver, a rod orother structural element, and a compression structure disposed betweenthe shank and the rod for holding the shank at a desired angle ofinclination or articulation with respect to the receiver; providing suchan assembly that has a low profile after final installation; providingsuch an assembly in which the compression structure may be inserted intoa bone screw receiver prior to installing the bone screw into bone;providing such an assembly in which an upper shank portion of the bonescrew includes a non-slip feature for driving the shank into bone;providing such an assembly in which an upper portion of the bone screwshank has a maximum diameter or width that is smaller than a diameter orwidth of a lower opening of the bone screw receiver and further includesan independent retaining and articulating structure fixable to the shankupper portion within the bone screw receiver; and providing such anassembly that is easy to use, especially adapted for the intended usethereof and wherein the implant assembly components are comparativelyinexpensive to produce.

Other objects and advantages of this invention will become apparent fromthe following description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of this invention.

The drawings constitute a part of this specification and includeexemplary embodiments of the present invention and illustrate variousobjects and features thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of an assembly according to theinvention including a shank with a capture structure at one end thereof,a receiver, a retaining and articulating structure and a compressionstructure.

FIG. 2 is an enlarged front elevational view of the compressionstructure of FIG. 1.

FIG. 3 is an enlarged top plan view of the compression structure of FIG.1.

FIG. 4 is an enlarged bottom plan view of the compression structure ofFIG. 1.

FIG. 5 is an enlarged, exploded front elevation of the shank andretaining and articulating structure of FIG. 1.

FIG. 6 is a partial cross-sectional view of the shank and retaining andarticulating structure taken along the line 6-6 of FIG. 5.

FIG. 7 is an enlarged and partial view of the shank and retaining andarticulating structure of FIG. 6.

FIG. 8 is an enlarged and partial cross-sectional view of the receivertaken along the line 8-8 of FIG. 1 and showing a first stage ofinsertion of the compression structure.

FIG. 9 is an enlarged and partial cross-sectional view of the receiversimilar to FIG. 8 and showing a fully installed compression structure.

FIG. 10 is a top plan view of the bone screw receiver, shank, retainingand articulating structure and compression structure of FIG. 9.

FIG. 11 is a cross-sectional view of the receiver, shank, retaining andarticulating structure and compression structure, shown being driveninto a vertebra with an Allen-type tool.

FIG. 12 is a partial cross-sectional view similar to FIG. 11 furthershowing a rod and a partially installed closure structure, also incross-section.

FIG. 13 is a partial cross-sectional view similar to FIG. 12 showing abreak-off head of the closure structure removed.

FIG. 14 is an enlarged and partial cross-sectional view taken along theline 14-14 of FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure.

With reference to FIGS. 1-14, the reference numeral 1 generallydesignates a polyaxial bone screw assembly according to the presentinvention. The assembly 1 includes a shank 4 that further includes abody 6 integral with an upwardly extending, substantially cylindricalend or capture structure 8; a receiver or head 10; a retaining andarticulating structure 12; and a compression structure 14. The shank 4,the receiver 10, the retaining and articulating structure 12 and thecompression structure 14 are preferably assembled prior to implantationof the shank body 6 into a vertebra 15, which procedure is shown in FIG.11 and will be discussed more fully below.

FIGS. 12-14 further show a closure structure generally 18, of theinvention for capturing a longitudinal member such as a rod 21 withinthe receiver 10. Upon installation, which will be described in detailbelow, the closure structure 18 presses against the rod 21 that in turnpresses against the compression structure 14 that presses against theretaining and articulating structure 12 that is threadably mated to thecapture structure 8. The compression structure 14 biases the retainingand articulating structure 12 into fixed frictional contact with thereceiver 10, so as to fix the rod 21 relative to the vertebra 15. Thereceiver 10, shank 4, retaining and articulating structure 12 andcompression structure 14 cooperate in such a manner that the receiver 10and shank 4 can be secured at any of a plurality of angles,articulations or rotational alignments relative to one another andwithin a selected range of angles both from side to side and from frontto rear, to enable flexible or articulated engagement of the receiver 10with the shank 4 until both are locked or fixed relative to each other.

The shank 4, best illustrated in FIGS. 1 and 5-7, is elongate, with theshank body 6 having a helically wound, radially outwardly extending boneimplantable thread 22 axially extending from near a tip 24 of the body 6to near a slanted or sloped surface 26 that is adjacent to a lateralprojection illustrated as a smooth cylindrical surface 28 locatedadjacent to the capture structure 8. As will be described more fullybelow, the laterally projecting cylindrical surface 28 includes abuttress stop feature 30 for frictional engagement with and placement ofthe retaining and articulating structure 12. During use, the body 6utilizing the thread 22 for gripping and advancement is implanted intothe vertebra 15 leading with the tip 24 and driven down into thevertebra 15 with an installation or driving tool so as to be implantedin the vertebra 15 to near the sloped surface 26, as shown in FIGS.12-14, and as is described more fully in the paragraphs below. The shank4 has an elongate axis of rotation generally identified by the referenceletter A. It is noted that any reference to the words top, bottom, upand down, and the like, in this application refers to the alignmentshown in the various drawings, as well as the normal connotationsapplied to such devices, and is not intended to restrict positioning ofthe assembly 1 in actual use.

The sloped surface 26 extends radially outward and axially upward fromthe shank body 6 to the cylindrical projection 28. Further extendingaxially from the projection 28 is the capture structure 8 that providesa connective or capture apparatus disposed at a distance from thethreaded shank body 6 and thus at a distance from the vertebra 15 whenthe body 6 is implanted in the vertebra 15.

The capture structure 8 is configured for connecting the shank 4 to thereceiver 10 and capturing the shank 4 in the receiver 10. The capturestructure 8 has an outer substantially cylindrical surface 34 having ahelically wound guide and advancement structure thereon which in theillustrated embodiment is a V-shaped thread 36 extending from adjacentthe cylindrical surface 28 to adjacent an annular upper surface 38. Theupper surface 38 is disposed substantially perpendicular to the axis ofrotation A. A diameter of the cylindrical surface 34 measured betweenroots of the thread 36 is smaller than a diameter of the projectedcylindrical surface 28. A diameter measured between crests of the thread36 is illustrated equal to and may be smaller than the diameter of thecylindrical surface 28. Although a simple thread 36 is shown in thedrawings, it is foreseen that other structures including other types ofthreads, such as buttress, square and reverse angle threads, and nonthreads, such as helically wound flanges with interlocking surfaces, maybe alternatively used in place of the thread 36 in alternativeembodiments of the present invention.

With particular reference to FIG. 7, the buttress stop feature 30disposed near a base or bottom of the thread 36 is defined in part bythe cylindrical surface 28 and in part by an upper shoulder 40 disposedperpendicular to the surface 28 and extending inwardly radially towardthe thread 36. In a preferred embodiment, a buttress stop feature 42disposed on the retaining and articulating structure 12 cooperates withthe feature 30 as will be described more fully below to stop theadvancement of the structure 12 along the thread 36 and provide for adesired placement of the structure 12 with respect to the capturestructure 8.

A driving formation 44 extends from the upper surface 38 into thecapture structure 8. The illustrated formation 44 includes six walls orfacets 46 disposed parallel to the axis A and a hex-shaped seatingsurface or base 48 disposed perpendicular to the axis A. The drivingformation 44 is sized and shaped to cooperate with a hex-driver forrotating and driving the shank body 6 into bone. It is foreseen thatother driving features or apertures, such as slotted, tri-wing,hexalobular (such as the 6-point star shaped pattern sold under thetrademark TORX), spanner, or the like may also be utilized according tothe invention.

In the illustrated embodiment, the shank 4 is cannulated with a smallcentral bore 49 extending an entire length of the shank along axis A.The bore 49 is coaxial with the threaded body 6 and the capturestructure outer surface 34, providing a passage through the shankinterior for a length of wire or pin inserted into the vertebra 15 priorto the insertion of the shank body 6, the wire or pin providing a guidefor insertion of the shank body 6 into the vertebra 15.

Referring to FIGS. 1, and 8-10, the receiver 10 has a generallycylindrical outer profile with a substantially cylindrical base 50integral with a pair of opposed upstanding arms 52 that extend from thebase 50 to a top surface 54. The arms 52 form a U-shaped cradle anddefine a U-shaped channel 56 between the arms 52 and include an upperopening 57 and a lower seat 58 having substantially the same radius asthe rod 21 for operably snugly receiving the rod 21.

Each of the arms 52 has an interior surface 60 that defines an innercylindrical profile and includes a partial helically wound guide andadvancement structure 62. In the illustrated embodiment, the guide andadvancement structure 62 is a partial helically wound flangeformconfigured to mate under rotation with a similar structure on theclosure top 18, as described more fully below. However, it is foreseenthat the guide and advancement structure 62 could alternatively be abuttress thread, a square thread, a reverse angle thread or other threadlike or non-thread like helically wound advancement structures foroperably guiding under rotation and advancing the closure top 18downward between the arms 52 and having such a nature as to resistsplaying of the arms 52 when the closure top 18 is advanced into theU-shaped channel 56.

Tool engagement apertures 64 are formed on outer substantiallycylindrical surfaces 66 of the arms 52 which may be used for holding thereceiver 10 with a holding tool (not shown) having projections that arereceived within the apertures 64 during implantation of the shank body 6into the vertebra 15. The apertures 64 may also cooperate with a holdingtool during bone screw assembly and during subsequent installation ofthe rod and closure top. The illustrated apertures 64 are circular anddisposed centrally on each arm 52. However, it is foreseen that theapertures may be configured in a variety of shapes and sizes and includeundercut surfaces and be disposed at other locations on the arms 52,including near the top surfaces 54. Also, the holding tool (not shown)and respective apertures 64 may be configured to provide for a varietyof ways to install the holding tool in the apertures, including a twiston/twist off engagement with the receiver, a twist on/snap offengagement or a flexible snap on/snap off engagement wherein the holdingtool has legs which splay outwardly to position the tool for engagementin the apertures 64 or a combination thereof.

Communicating with the U-shaped channel 56 and located within the base50 of the receiver 10 is a chamber or cavity 78 partially defined by aninner cylindrical surface 80, the cavity 78 opening upwardly into theU-shaped channel 56. In the illustrated embodiment, the cylindricalsurface 80 has a diameter equal to an inner diameter between the arms 52measured between crests of the guide and advancement structure 62. Inthe illustrated embodiment, the cylindrical inner surface 80 terminatesat a ledge or lower shoulder 81 that is disposed perpendicular to anaxis of rotation B of the receiver. The shoulder 81 is adjacent to apartial internal spherical seating surface 82 having a first radius. Thesurface 82 is sized and shaped for mating with the retaining andarticulating structure 12, as described more fully below. It is foreseenthat the surface 82 may be partially spherical or conical, or the like,and may include a high friction surface.

The base 50 further includes a restrictive neck 83 adjacent the seatingsurface 82. The neck 83 defines a bore 84 communicating with the cavity78 and a lower exterior 86 of the base 50. The bore 84 is coaxiallyaligned with respect to the rotational axis B of the receiver 10. Thebore 84 may be conically counterbored or beveled in a region 87 to widenthe angular range of the shank 4. The neck 83 and associated bore 84 aresized and shaped to be smaller than a radial dimension of a fixed orfully expanded retaining and articulating structure 12, as will bediscussed further below, so as to form a restriction at the location ofthe neck 83 relative to the retaining and articulating structure 12, toprevent the structure 12 from passing from the cavity 78 and out intothe lower exterior 86 of the receiver 10 when the retaining andarticulating structure 12 is seated on the seating surface 82. Again, itis foreseen that the retaining and articulating structure could becompressible (such as where such structure has a missing section) andcould be loaded through the neck 83 and then allowed to expand and fullyseat in the spherical seating surface 82.

The retaining and articulating structure 12, best illustrated by FIGS. 1and 5-7, has an operational central axis that is the same as theelongate axis A associated with the shank 4, but when the structure 12is separated from the shank 4, the axis of rotation is identified as anaxis C. The retaining and articulating structure 12 has a central bore90 that passes entirely through the structure 12 from a top surface 92to a bottom surface 94 thereof. An inner cylindrical surface 96 definesa substantial portion of the bore 90, the surface 96 having a helicallywound guide and advancement structure thereon as shown by a v-shapedhelical rib or thread 98 extending from adjacent the top surface 92 tonear the bottom surface 94. Although a simple helical rib 98 is shown inthe drawings, it is foreseen that other helical structures includingother types of threads, such as buttress and reverse angle threads, andnon threads, such as helically wound flanges with interlocking surfaces,may be alternatively used in an alternative embodiment of the presentinvention. The inner cylindrical surface 96 with the thread 98 areconfigured to mate under rotation with the capture structure outersurface 34 and helical guide and advancement structure or thread 36, asdescribed more fully below.

The buttress stop formation 42 of the retaining and articulatingstructure 12 that is sized and shaped to mate with the stop 30 locatedon the shank 4 is located axially between the helical rib 98 and thebottom surface 94 of the structure 12. The formation 42 includes a lowershoulder 100 extending radially from the thread 98 and towards thestructure 12 and a cylindrical wall 102 disposed perpendicular to thelower shoulder 100. The lower shoulder 100 is sized and shaped to mateand abut with the upper shoulder 40 and the cylindrical wall 102 issized and shape to mate with the cylindrical projection 28. Thus, aswill be described in more detail below, when the retaining andarticulating structure 12 is rotated and mated with the capturestructure 8 and fully installed thereon, the lower shoulder 100 of thestructure 12 abuts the upper shoulder 40 of the stop 30. The retainingand articulating structure 12 and the capture structure 8 are configuredsuch that when the buttress stop 30 abuts the buttress stop 42, the topsurface 92 of the structure 12 is flush with the upper surface 38 of thecapture structure 8. A sloped surface or chamfer 103 runs between thecylindrical wall 102 and the bottom surface 94 of the retaining andarticulating structure 12.

It is foreseen that other types of geometrical orientation or structuremay be utilized to engage or mate the capture structure and theretaining and articulating structure. For example, the capture structuremay have an outer surface that is frusto-conical and the retaining andarticulating structure may be a split ring with an inner surface sizedand shaped to frictionally engage the frusto-conical capture structure.Also, the capture structure may have an inverted polyhedral or conicalgeometry and the mating retaining and articulating structure may be aplurality of pieces, the geometry of the pieces corresponding andcooperating with the polyhedral or conical geometry of the capturestructure to frictionally envelope the retaining and articulatingstructure between the capture structure and an internal surface defininga cavity of the receiver.

The illustrated retaining and articulating structure 12 has a radiallyouter partially spherically shaped surface 104 sized and shaped to matewith the partial spherically shaped seating surface 82 of the receiverand having a radius approximately equal to the radius associated withthe surface 82. The retaining and articulating structure radius islarger than the radius of the neck 83 of the receiver 10. Although notrequired, it is foreseen that the outer partially spherically shapedsurface 104 may be a high friction surface such as a knurled surface orthe like.

It is also foreseen that the retaining and articulating structure outersurface may be elliptical or ellipsoid in shape rather than spheroid inshape. Such an elliptical surface would be sized and shaped to contactand seat within a substantially spherical seating surface, such as theseating surface 82. Such an ellipsoid structure may be attachable to theshank upper portion by threads, a pin, compression, or the like aspreviously described with respect to the substantially sphericalretaining and articulating structure 12. Furthermore, it is foreseenthat an ellipsoid retaining structure may be integral with the bonescrew shank and may include threads that allow the ellipsoid to bethreadably received into a base of a bone screw receiver.

The illustrated retaining and articulating structure top surface 92extends from the central bore 90 to the outer surface 104. The topsurface 92 is disposed perpendicular to the axis of rotation C of thestructure 12. The bottom surface 94 extends from the chamfer 103 to theouter surface 104 and also is disposed perpendicular to the axis ofrotation C.

The elongate rod or longitudinal member 21 that is utilized with theassembly 1 can be any of a variety of implants utilized inreconstructive spinal surgery, but is normally a cylindrical elongatestructure having a smooth, outer cylindrical surface 108 of uniformdiameter. The rod 21 is preferably sized and shaped to snugly seat nearthe bottom of the U-shaped channel 56 of the receiver 10 and, duringnormal operation, is positioned slightly above the bottom of the channel56 at the lower seat 58.

The compression structure 14 is best illustrated in FIGS. 1-4. In theembodiment shown, the compression structure 14 includes a body 110 ofsubstantially circular cross-section integral with a pair of upstandingarms 112. The body 110 and arms 112 form a generally U-shaped, open,through-channel 114 having a substantially U-shaped bottom seatingsurface 116 having a radius substantially conforming to a radius of therod 21 and thus configured to operably snugly engage the rod 21. Thearms 112 disposed on either side of the channel 114 each included a topsurface 118 that is parallel to an annular bottom surface 120. Thecompression structure 14 includes a substantially cylindrical outersurface 122 and an inner cylindrical wall 124 defining a centralthrough-bore 125 extending along a central axis D of the compressionstructure 14. The top surface 118 and the bottom surface 120 aredisposed perpendicular to the axis D. Extending between the innercylindrical wall 124 and the bottom surface 120 is a curved or sphericalsurface 126 sized and shaped to frictionally engage and mate with theouter spherical surface 104 of the retaining and articulating structure12. The cylindrical surface 122 has a diameter slightly smaller than adiameter between crests of the guide and advancement structure 62allowing for top loading of the compression structure 14 as illustratedin FIGS. 8-10. The cylindrical surface 122 diameter and a height of thecompression structure 14 measured from the top surface 118 to the bottomsurface 120 are sized such that the compression structure 14 is receivedwithin the cylindrical surface 80 of the receiver 10 below the guide andadvancement structure 62, but the bottom surface 120 thereof does notengage the ledge 81 when fully installed on the retaining andarticulating structure 12. There is thus a space between the bottomsurface 120 and the ledge 81 in any angular position of the shank 4 withrespect to the receiver 10. When fully installed, the compressionstructure 14 does not contact the bone screw shank capture structure 8,but engages only with the retaining and articulating structure 12. Whenpressed upon by the rod 21, the surface 126 of the compression structure14 frictionally engages the surface 104 of the retaining andarticulating structure 12, which in turn presses upon the seatingsurface 82 of the receiver 10. In some embodiments, the compressionstructure could be up-loaded into the receiver, followed by up-loadingof the retaining structure into the receiver.

With particular reference to FIGS. 12-14, the closure structure 18 canbe any of a variety of different types of closure structures for use inconjunction with the present invention with suitable mating structure onthe upstanding arms 52 of the receiver 10. The closure structure 18 isrotatable between the spaced arms 52. The illustrated structure 18includes a cylindrical base 140 and a break-off head 142. Helicallywound about the base 140 is a guide and advancement structure in theform of a flange form 144. The illustrated guide and advancementstructure 144 operably joins with the guide and advancement structure 62disposed on the interior 60 of the arms 52. The flange form 144 includesa root 146 and a crest 148. Furthermore, the flange form 144 also has atrailing surface 150 and a leading surface 152 which are relative to theforward movement of the closure 18 as it is rotated clockwise about thecentral axis B of the bone screw receiver and joined therewith. Locatedon the trailing surface 150 or the leading surface 152 or both is aprojection which protrudes rearwardly or frontwardly with respect to thewidth of the flange form 144 at the root 146 and which interlocks withthe guide and advancement mating structure 62 of the receiver 10.

In the illustrated embodiment, the flange form 144 has a protrusion 154that projects rearwardly from the trailing surface 150. The flange form144 utilized in accordance with the present invention may be anystructure which effectively locks the closure 18 to the structure withinwhich it is set so as to prevent splaying of the structure upon whichmating guide and advancement structure is mounted. Various flange formstructures which can be used alternatively are illustrated inapplicant's U.S. Pat. No. 6,726,689, which is incorporated herein byreference. As stated herein with respect to the flange form guide andadvancement structure 62, it is also foreseen that according to theinvention the guide and advancement structure 144 could alternatively bea buttress thread, a square head, a reverse angle thread or other threadlike or non-thread like helically wound advancement structure foroperably guiding under rotation and advancing the closure 18 downwardbetween the arms 52 and having such a nature as to resist splaying ofthe arms 52 when the closure top 18 is advanced into the U-shapedchannel 56.

The base 140 of the closure structure 18 includes a lower surface 156having a dome 158 located thereon. The dome 158 extends greatest fromthe base 140 along a central axis E that is operably coaxial with thereceiver axis B. The dome 158 in the present embodiment is spherical inshape and, in particular, is a partial sphere that has a uniform orconstant radius of generation.

However, it is foreseen that in certain embodiments the radius may varydepending upon the needs and desires of the particular structure and thedome 158 may have shape that is only partly a spherical curved surfaceor some other shape. The dome 158 may be a simple curved surface thatallows greatest projection along the axis. That is, the dome surfacecould be radiused at the location of greatest projection and featheredalong the periphery so as to not have a continuous uniform radius ofgeneration throughout, but rather a continually changing radius ofgeneration along at least the length thereof. Preferably, the dome 158is smoothly curved where the dome 158 intersects with the axis E. It isalso foreseen that the lower surface 156 could be flat or have a pointand rim geometry.

The closure structure 18 break off head 142 is secured to the base 140by a break off region 160 that is designed to allow the head 142 tobreak from the base 140 at a preselected torque, for example, 70 to 140inch pounds. The break off head 142 has an external radial outwardsurface with six planar facets 162 so as to form a structure designed tobe received within a socket of a driving type tool (not shown) with asimilar receiving shape. The break off head 142 has a central bore 164that may also include driving formations suitable for engagement by atool (not shown).

During installation, the dome 158 engages the rod 21 at an apex 166 asseen in FIGS. 13 and 14. The closure structure 18 is torqued until apreselected pressure is reached at which point the closure 18 at theapex 166 abuts the rod 21 which in turn is urged toward but notcompletely to the lower seat 58 of the channel 56. In turn, the rod 21braces against the compression structure 14 which urges the retainingand articulating structure 12 to fixedly seat in the cavity 78.Thereafter, the receiver 10 is no longer rotatable relative to the shank4, but rather is locked in position.

The closure structure 18 also includes removal tool engagement structurewhich in the present embodiment is in the form of a hex-shaped andaxially aligned aperture 168 disposed in the base 140, as shown in FIG.13. The hex aperture 168 is accessible after the break-off head 142breaks away from the base 140. The aperture 168 is coaxial with thehelically wound guide and advancement structure 144 and is designed toreceive a hex tool, of an Allen wrench type, into the aperture 168 forrotating the closure structure base 140 subsequent to installation so asto provide for removal thereof, if necessary. Although a hex-shapedaperture 168 is shown in the drawings, the tool engagement structure maytake a variety of tool-engaging forms and may include more than oneaperture of various shapes, such as a pair of spaced apertures, a lefthand threaded bore, an easy out engageable step down bore or the like.

With reference to FIG. 1, prior to the polyaxial bone screw assembly 1being implanted in the vertebra 15, the retaining and articulatingstructure 12 is typically first inserted or top-loaded, into thereceiver U-shaped channel 56, and then into the cavity 78 to dispose thestructure 12 adjacent the inner surface 80 of the receiver 10. Thestructure 12 may be loaded with the axis C coaxial with the receiveraxis B or turned or rotated such that the axis C is perpendicular to theaxis B of the receiver 10 during insertion of the structure 12 into thereceiver 10. Then, after the retaining and articulating structure 12 iswithin the cavity 78, the retaining and articulating structure 12 isrotated approximately 90 degrees such that the axis C is coaxial withthe axis B of the receiver 10, and then the structure 12 is seated insliding engagement with the seating surface 82 of the receiver 10.

The shank capture structure 8 is preloaded, inserted or bottom-loadedinto the receiver 10 through the bore 84 defined by the neck 83. Theretaining and articulating structure 12, now disposed in the receiver 10is coaxially aligned with the shank capture structure 8 so that thehelical v-shaped thread 36 rotatingly mates with the thread 98 of theretaining and articulating structure 12.

With reference to FIGS. 5-7, the shank 4 and/or the retaining andarticulating structure 12 are rotated to fully mate the structures 36and 98 along the respective cylindrical surfaces 34 and 96, fixing thecapture structure 8 to the retaining and articulating structure 12,until the lower shoulder 100 of the buttress stop 42 abuts the uppershoulder 40 of the stop 30.

With reference to FIGS. 8-10, at this time the shank 4 is in slidableand rotatable engagement with respect to the receiver 10, while thecapture structure 8 and the lower aperture or neck 83 of the receiver 10cooperate to maintain the shank body 6 in rotational relation with thereceiver 10. According to the illustrated embodiment, only the retainingand articulating structure 12 is in slidable engagement with thereceiver spherical seating surface 82. Both the capture structure 8 andthreaded portion of the shank body 6 are in spaced relation with thereceiver 10. The shank body 6 can be rotated through a substantialangular rotation relative to the receiver 10, both from side to side andfrom front to rear so as to substantially provide a universal or balljoint wherein the angle of rotation is only restricted by engagement ofthe neck 26 of the shank body 6 with the neck or lower aperture 83 ofthe receiver 10.

In the embodiment shown, the compression structure 14 illustrated inFIGS. 1-4 is then loaded into the receiver 10 as illustrated in FIGS.8-10. With particular reference to FIG. 8, the insert U-shaped channel114 is aligned with the receiver 10 U-shaped channel 56 and thecompression structure 14 is initially top or down-loaded into thereceiver 10 until the arms 112 are disposed adjacent to the surface 80and the bottom spherical surface 126 is in contact with the surface 104of the retaining and articulating structure 12. To ready the assembly 1for implantation into bone, the shank 4, receiver 10 and compressionstructure 14 axes A, B and D, respectively are aligned, providing accessto the hex-shaped formation 44 on the shank capture structure 8 throughthe bore 125 of the compression structure 14. Such placement allows forunrestricted rotation of the shank body 6 with respect to the receiver10.

With reference to FIG. 11, the assembly 1 is typically screwed into abone, such as the vertebra 15, by rotation of the shank 4 using adriving tool with an Allen type driving formation 175 that operablydrives and rotates the shank 4 by engagement thereof with the shank atthe driving formation 44, a base 177 of the tool 175 abutting andengaging the driving formation 44 at the base 48 thereof. It is foreseenthat in other embodiments according to the invention, the hex-shapeddriving formation 44 may be replaced by other types of foot print typetool engaging formations or recesses. Through the driving formationaperture, the retaining structure and the shank can be crimped togetherso as to not come apart with rotation.

Typically at least two and up to a plurality of bone screw assemblies 1are implanted into vertebrae for use with the rod 21. Each vertebra 15may be pre-drilled to minimize stressing the bone. Furthermore, when acannulated bone screw shank is utilized, each vertebra will have a guidewire or pin (not shown) inserted therein that is shaped for the bonescrew cannula 49 of the bone screw shank and provides a guide for theplacement and angle of the shank 4 with respect to the vertebra 15. Afurther tap hole may be made using a tap. The shank body 6 is thendriven into the vertebra 15, by rotation of the driving tool 175.

With reference to FIGS. 12-14, the rod 21 is eventually positionedwithin the receiver U-shaped channel 56, and the closure structure isthen inserted into and advanced between the arms 52. The compressionstructure 14 is pressed downwardly into engagement with the retainingand articulating structure outer surface 104 to set the angle ofarticulation of the shank body 6 with respect to the receiver 10 bypressure from the rod 21 that in turn is being pressed upon by the dome158 of the closure structure 18. The rod 21 is seated on the compressionstructure 14 and the fastener 18 is initially placed between the arms 52and rotated using an installation tool (not shown) engaged with thesurfaces 162 of the break-off head 142 until the guide and advancementstructure 144 is fully mated with the receiver guide and advancementstructure 62. With reference to FIG. 13, the break-off head 142 is thentwisted to a preselected torque, for example 70 to 140 inch pounds, alsoutilizing the installation tool in engagement with the faceted outersurface 162 of the break-off head 142, with or without bending of therod 21 in order to achieve and maintain a desired alignment of thespine. As illustrated in FIGS. 13 and 14, upon final installation, astable fixation of the rod 21 is accomplished with one area of contactprovided at the apex 166 of the closure top dome 158 and two areas ofcontact provided between the rod 21 and the compression structure 14.

If removal of the assembly 11 is necessary, or if it is desired torelease the rod 21 at a particular location, disassembly is accomplishedby using an Allen type tool (not shown) with the hex-shaped drivingformation 168 located on the closure structure base 140 to rotate andremove the closure structure base 140 from the receiver 10. Disassemblyof the assembly 1 is accomplished in reverse order to the proceduredescribed previously herein for assembly. Again, it is foreseen that anon-break off closure could be used which is inserted and removed withthe same driving formation.

It is to be understood that while certain forms of the present inventionhave been illustrated and described herein, it is not to be limited tothe specific forms or arrangement of parts described and shown.

What is claimed is:
 1. A pivotal bone anchor assembly configured for implantation into a bone of a patient with a drive tool and for securing a fixation rod to the bone with a fastener configured to apply a downwardly-directed force to lock the assembly, the pivotal bone anchor assembly comprising: a receiver comprising a base defining a central bore centered about a vertical centerline axis and an upper portion defining a through channel configured to receive the fixation rod, the central bore communicating with a bottom of the receiver through a lower opening and extending upward through the channel to a top of the receiver and including a partially spherical engagement surface adjacent the lower opening and a guide and advancement structure adjacent the top of the receiver configured to mate with the fastener; a shank comprising a longitudinal axis, an anchor portion at a distal end configured for fixation to the bone, and a capture portion at a proximal end including an upwardly-opening internal drive socket configured to mate with the drive tool and a partially spherical lower surface curving upwardly and outwardly from a neck portion that extends between the capture portion and the anchor portion, the capture portion being uploadable into the central bore of the receiver through the lower opening; a retainer comprising a compressible, open ring-shaped body having at least a slit or missing section, a center opening, and an outer partially spherical surface configured for pivotal engagement with the partially spherical engagement surface of the receiver, the retainer being configured for positioning into the lower portion of the central bore prior to the capture portion of the shank and for capturing the capture portion upon its uploading through the lower opening so as to pivotally hold the capture portion of the shank in the receiver; and an insert positionable within the central bore of the receiver comprising a center aperture configured to provide access to the internal drive socket for the drive tool, an upper surface configured to engage the fixation rod, and a lower surface configured to transfer the downwardly-directed force toward the capture portion of the shank to lock the assembly.
 2. The pivotal bone anchor assembly of claim 1, wherein the capture portion of the shank is configured for positioning within the receiver prior to the insert.
 3. The pivotal bone anchor assembly of claim 1, wherein the capture portion of the shank includes a threaded outer surface configured to threadably mate under rotation with a threaded inner surface of the center opening of the retainer.
 4. The pivotal bone anchor assembly of claim 1, wherein the capture portion of the shank includes a frusto-conical outer surface configured to frictionally mate with a complementary-shaped inner surface of the center opening of the retainer.
 5. The pivotal bone anchor assembly of claim 1, wherein a top surface of the capture portion of the shank does not extend above a top surface of the retainer.
 6. The pivotal bone anchor assembly of claim 1, wherein the internal drive socket further comprises an internal drive aperture with interior sidewalls extending downward from a top of the capture portion to a bottom abutment surface configured for engagement by the drive tool.
 7. The pivotal bone anchor assembly of claim 6, wherein the shank includes an axial bore centered about the longitudinal axis and extending distally from an upper opening communicating with the bottom abutment surface of the internal drive aperture to a distal tip of the anchor portion.
 8. The pivotal bone anchor assembly of claim 7, wherein the axial bore of the shank defines a closed surface along an entire length thereof.
 9. The pivotal bone anchor assembly of claim 1, wherein the capture portion of the shank includes a planar annular top surface perpendicular to the longitudinal axis surrounding and entirely peripheral to the internal drive socket.
 10. The pivotal bone anchor assembly of claim 9, wherein the planar annular top surface includes a substantially circular outer edge.
 11. The pivotal bone anchor assembly of claim 1, wherein the neck portion of the shank has a substantially smooth outer surface.
 12. The pivotal bone anchor assembly of claim 1, wherein the through channel of the receiver further comprises an upwardly-open channel defined by inner surfaces of a pair of upright arms extending upwardly from the base.
 13. The pivotal bone anchor assembly of claim 12, wherein the guide and advancement structure of the central bore of the receiver further comprises a discontinuous helically wound guide and advancement structure configured to resist splaying of the pair of upright arms upon mating with the fastener.
 14. The pivotal bone anchor assembly of claim 13, wherein the discontinuous helically wound guide and advancement structure is a buttress thread.
 15. The pivotal bone anchor assembly of claim 13, wherein the discontinuous helically wound guide and advancement structure is a square thread.
 16. The pivotal bone anchor assembly of claim 13, wherein the discontinuous helically wound guide and advancement structure is a reverse angle thread.
 17. The pivotal bone anchor assembly of claim 1, wherein the insert is spaced apart from the fastener when the fixation rod is positioned within the through channel of the receiver and against the upper surface of the insert and the assembly is locked with the fastener.
 18. The pivotal bone anchor assembly of claim 1, wherein the upper surface of the insert further comprises a U-shaped through channel defined by a pair of upstanding arms, the upstanding arms having uppermost top surfaces configured to be positioned below a midpoint of the fixation rod when the fixation rod is positioned within the channel of the receiver and against the U-shaped through channel of the insert and the assembly is locked with the fastener.
 19. The pivotal bone anchor assembly of claim 1, wherein the insert is configured to have a non-snap-fit engagement with the fixation rod.
 20. The pivotal bone anchor assembly of claim 1, wherein the lower surface of the insert is configured to engage the outer partially spherical surface of the retainer.
 21. The pivotal bone anchor assembly of claim 1, wherein the lower surface of the insert is spaced from the capture portion of the shank throughout all articulations of the shank relative to the receiver.
 22. The pivotal bone anchor assembly of claim 1, wherein the partially spherical lower surface of the capture portion of the shank extends outwardly and upwardly from the neck portion to define a partially spherical outer surface formed integral with the shank and having a diameter great than a diameter of the lower opening so as to maintain the capture portion within the central bore of the receiver with the shank extending downward through the lower opening.
 23. The pivotal bone anchor assembly of claim 1 and further comprising the fixation rod and the fastener, wherein the fastener is configured for positioning within the central bore of the receiver above the fixation rod positioned in the through channel, and for engagement with the guide and advancement structure to apply the downwardly-directed force towards a top of the fixation rod so as to frictionally lock the shank in a fixed position relative to the receiver. 